Wednesday, August 11, 2010

Amide hydrolysis, revisited 2

In a previous post I discussed the TSs of acetamide hydrolysis and hydrolysis of a related amide (3). I have already made a post on how to find the TS for 3, and in this post I summarize the two ways of finding the TS for acetamide hydrolysis, that I describe in Chapter 5 of the book (where you can find many more details).

To start, I use Avogadro to build the geometry shown in Figure 5.32a and use it to construct the input file for an optimization with 4 constrained bond length. The values for the constraints are taken from a paper. Figure 5.32b shows the equilibrium geometry obtained with GAMESS, after 17 steps. Based on this geometry GAMESS finds the TS in three steps.

It will not always be possible to find an article that reports the TS structure for your reaction of interest. So let’s try to find the TS without the information from the article. I start from the same structure (Figure 5.32a), and I arbitrarily pick C1-O10 as the reaction coordinate constrained to 1.70 Å.

This constrained optimization results (after 23 steps) in the geometry shown in Figure 5.35a. The subsequent TS search results in the geometry shown in Figure 5.35b, which has no imaginary frequency and is clearly the product.

Figure 5.35a. The geometry resulting from a constrained optimization in which the distance between atoms 1 and 10 (see Figure 5.32a for numbering) was constrained to 1.7 Å, and the normal mode of the imaginary frequency computed for this structure at the PM3 level.

The fact that the C–O bond is re-formed indicates that it should be stretched more in the initial guess, so I repeat the optimization with the C–O distance constrained to 2.0 Å instead of 1.70 Å. This results (after 39 steps) in the geometry shown in Figure 5.36. Using this as a starting geometry for the TS search leads to the TS in Figure 4.30a after 17 steps.

Figure 5.36. The geometry resulting from a constrained optimization in which the distance between atoms 1 and 10 (see Figure 5.32a for numbering) was constrained to 2.0 Å, and the normal mode of the imaginary frequency computed for this structure at the PM3 level.

In the screencast below I try to reproduce the calculations that lead to these figures. Because I rebuild the structure in Figure 5.32a, I get different starting coordinates, so the energies and number of steps are different than what I discuss in the book for the 4-constraints approach.

In the case of the 1-constraint approach, I actually manage to find the TS using the 1.70 Å constraint. Thus, the structures in Figures 5.35b and 5.36 do not appear in the screencast. This just goes to show how finicky TS searchers are to starting geometries.

While editing the screencast I noticed that the PM3 energies of the two TS structures I find are very different (11 kcal/mol). This is also true for the TSs I found when writing the book (where they are different by 6.6 kcal/mol).

This appears to be a problem that PM3 has with structures where bonds are partially broken or formed. I could not reproduce this for structures with normal bond lengths such as the reactants and products. Note that in the book, I use M06/6-31G(d) single point energies to compute barriers, and that the M06/6-31G(d)//PM3 single point energies of the TSs found with the two different method are only different by 0.03 kcal/mol.